Optical Signal to Noise Ratio (OSNR) refers to the ratio of an optical signal power to noise signal power in an optical network. OSNR is a key figure of merit for optical networks. In conventional optical networks, OSNR is calculated at the time of set up for the optical networks. In other words, the OSNR is calculated when an optical network is being configured for initial service. For a given link or a set of links in an optical network, the OSNR measurement is done before the link is in-service through an external test because the signal level and the noise level need to be measured separately. In order to be able to measure the noise level accurately, the signal channel is turned off so as to be able to evaluate the true in-band noise level. This conventional approach is service interrupting such that it cannot be conducted in-service.
The conventional one-time beginning-of-line OSNR measurement of a link fails to account for events occurring during operation that can affect the OSNR value. For example, amplifiers may degrade over time when the optical network is in-service. The degradation is not accounted for in the conventional OSNR measurements. In addition, optical networks may be built with add/drop nodes where some waves are transparently passed through while other waves are terminated. Conventional OSNR measurement techniques rely on segment-by-segment OSNR measurements taken at the initial time of install. End-to-end link OSNR is inferred using the segment-by-segment OSNR measurements. This approach is often error prone and practically infeasible if one segment is already in-service with waves transmitting.